1. The Field of the Invention
This invention pertains to methods and apparatus for processing digital images of vascular structures including blood vessels. More particularly, it relates to methods for interpreting ultrasonic images of the common carotid artery.
2. The Background Art
Coronary artery disease (CAD) is a narrowing of the arteries that supply the heart with blood carrying oxygen and nutrients. CAD may cause shortness of breath, angina, or even heart attack. The narrowing of the arteries is typically due to the buildup of plaque, or, in other words, an increase in the atherosclerotic burden. The buildup of plaque may also create a risk of stroke, heart attacks, and embolisms caused by fragments of plaque detaching from the artery wall and occluding smaller blood vessels. The risk of plaque detachment is particularly great when it is first deposited on the artery wall inasmuch as it is soft and easily fragmented at that stage.
Measurement of the atherosclerotic burden of the coronary artery itself is difficult and invasive. Moreover, assessment of risk often involves measuring both the atherosclerotic burden and its rate of progression. This assessment therefore involves multiple invasive procedures over time. Treatment of CAD also requires additional invasive procedures to measure a treatment""s effectiveness.
The carotid artery, located in the neck close to the skin, has been shown to mirror the atherosclerotic burden of the coronary artery. Moreover, studies have shown that a reduction of the atherosclerotic burden of the coronary artery will parallel a similar reduction in the carotid artery.
One noninvasive method for measuring the atherosclerotic burden is the analysis of ultrasound images of the carotid artery. High resolution, B-mode ultrasonography is one adequate method of generating such images. Ultrasound images typically provide a digital image of the various layers comprising the carotid artery wall, which may then be measured to determine or estimate the extent of atherosclerosis. Other imaging systems may likewise provide digital images of the carotid artery, such as magnetic resonance imaging (MRI) and radio frequency imaging.
The wall of the carotid artery comprises the intima, which is closest to the blood flow and which thickens, or appears to thicken, with the deposit of fatty material and plaque; the media, which lies adjacent the intima and which thickens as a result of hypertension; and the adventitia, which provides structural support for the artery wall. The channel in which blood flows is the lumen. The combined thickness of the intima and media layers, or intima-media thickness (IMT), is reflective of the condition of the artery and can accurately identify or reflect early stages of atherosclerotic disease.
An ultrasound image typically comprises an array of pixels, each with a specific value corresponding to its intensity. The intensity (brightness) of a pixel corresponds to the density of the tissue it represents, with brighter pixels representing denser tissue. Different types of tissue, each with a different density, are therefore distinguishable in an ultrasonic image. The lumen, intima, media and adventitia may be identified in an ultrasound image due to their differing densities.
An ultrasound image is typically formed by emitting sound waves toward the tissue to be measured and measuring the intensity and phase of sound waves reflected from the tissue. This method of forming images is subject to limitations and errors. For example, images may be subject to noise from imperfect sensors. Another source of error is the attenuation of sound waves that reflect off tissue located deep within the body or beneath denser tissue. Random reflections from various objects or tissue boundaries, particularly due to the non-planar ultrasonic wave, may add noise also.
The limitations of ultrasonography complicate the interpretation of ultrasound images. Other systems designed to calculate IMT thickness reject accurate portions of the image when compensating for these limitations. Some IMT measurement systems will divide an image into columns and examine each column, looking for maxima, minima, or constant portions of the image in order to locate the layers of tissue comprising the artery wall. Such systems may reject an entire column of image data in which selected portions of the wall are not readily identifiable. This method fails to take advantage of other portions of the artery wall that are recognizable in the column. Furthermore, examining columns of pixels singly fails to take advantage of accurate information in neighboring columns from which one may extrapolate, interpolate, or otherwise guide searches for information within a column of pixels.
Another limitation of prior methods is that they fail to adequately limit the range of pixels searched in a column of pixels. Noise and poor image quality can cause any search for maxima, minima, or intensity gradients to yield results that are clearly erroneous. Limiting the field of search is a form of filtering that eliminates results that cannot possibly be accurate. Prior methods either do not limit the field searched for critical points or apply fixed constraints that are not customized, or even perhaps relevant, to the context of the image being analyzed.
What is needed is a method for measuring the IMT that compensates for limitations in ultrasonic imaging methods. It would be an advancement in the art to provide an IMT measurement method that compensates for noise and poor image quality while taking advantage of accurate information within each column of pixels. It would be a further advancement to provide a method for measuring the IMT that limited the field of search for critical points to regions where the actual tissue or tissue boundaries can possibly be located.
In view of the foregoing, it is a primary object of the present invention to provide a novel method and apparatus for extracting IMT measurements from ultrasound images of the carotid artery.
It is another object of the present invention to reduce error in IMT measurements by restricting searches for the lumen/intima boundary and media/adventitia boundary to regions likely to contain them.
It is another object of the present invention to bound a search region using a datum, or datums, calculated beforehand based on analysis of a large portion of a measurement region in order to improve processing speed and accuracy.
It is another object of the present invention to validate putative boundary locations using thresholds reflecting the actual make-up of the image.
It is another object of the present invention to validate putative boundary locations based on their proximity to known features of ultrasound images of the carotid artery.
It is another object of the present invention to compensate for sloping and tapering of the carotid artery as well as misalignment of an image frame of reference with respect to the axial orientation of the artery.
It is another object of the present invention to compensate for low contrast and noise by extrapolating and interpolating from high contrast portions of an image into low contrast portions of the image.
Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, an apparatus is disclosed in one embodiment of the present invention as including a computer programmed to run an image processing application and to receive ultrasound images of the common carotid artery.
An image processing application may carry out a process for measuring the intima-media thickness (IMT) providing better measurements, less requirement for user skill, and a higher reproducibility. As a practical matter, intensity varies with the constitution of particular tissues. However, maximum difference in intensity is not typically sufficient to locate the boundaries of anatomical features. Accordingly, it has been found that in applying various techniques of curve fitting analysis and signal processing, structural boundaries may be clearly defined, even in the face of comparatively xe2x80x9cnoisyxe2x80x9d data.
In certain embodiments of a method and apparatus in accordance with the invention, an ultrasonic imaging device or other imaging devices, such as a magnetic resonance imaging system (MRI), a computed tomography scan (CT-Scan), a radio frequency image, or other mechanism may be used to create a digital image. Typically, the digital image contains various pixels, each pixel representing a picture element of a specific location in the image. Each pixel is recorded with a degree of intensity. Typical intensity ranges run through values between zero and 255. In alternative embodiments, pixels may have color and intensity.
In certain embodiments, an image is first calibrated for dimensions. That is, to determine an IMT value, the dimensions of the image must necessarily be calibrated against a reference measurement. Accordingly, the scale on an image may be applied to show two dimensional measurements across the image.
In certain embodiments, an ultrasonic image is made with a patient lying on the back with the image taken in a horizontal direction. Accordingly, the longitudinal direction of the image is typically horizontal, and coincides with approximately the axial direction of the carotid artery. A vertical direction in the image corresponds to the approximate direction across the carotid artery.
In certain embodiments of methods and apparatus in accordance with the invention, a measurement region may be selected by a user, or by an automated algorithm. A user familiar with the appearance of a computerized image from an ultrasound system may quickly select a measurement region. For example, the horizontal center of an image may be selected near the media/adventitia boundary of the blood vessel in question.
Less dense materials tend to appear darker in ultrasound images, having absorbed the ultrasonic signal from a transmitter, and thus provide less of a return reflection to a sensor. Accordingly, a user may comparatively quickly identify high intensity regions representing the more dense and reflective material in the region of the adventitia and the darker, low density or absorptive region in the area of the lumen.
In general, a method or characterizing plaque buildup in a blood vessel may include a measurement of an apparent intima-media thickness. In one embodiment, the method may include providing an image. An image is typically oriented with a longitudinal direction extending horizontally relative to a viewer and a transverse direction extending vertically relative to a viewer. This orientation corresponds to an image taken of a carotid artery in the neck of the user lying on an examination table. Thus, the carotid artery is substantially horizontally oriented. The axial direction is the direction of blood flow in a blood vessel, and the lateral direction is substantially orthogonal thereto. The image is typically comprised of pixels. Each pixel has a corresponding intensity associated with the intensity of the sound waves reflected from that location of the subject represented by a selected region of the image created by the received waves at the wave receiver.
In selected embodiments of an apparatus and method in accordance with the invention, a series of longitudinal positions along the image may be selected and the brightest pixel occurring in a search in the lateral direction is identified for each longitudinal position. The brightest pixel at any longitudinal position is that pixel, located in a lateral traverse of pixels in the image, at which the image has the highest level of intensity. The brightest pixels may be curve fit by a curve having a domain along the longitudinal direction, typically comprising the longitudinal locations or positions, and having a range corresponding to the lateral locations of each of the brightest pixels. A curve fit of these brightest pixels provides a curve constituting an adventitia datum.
The adventitia datum is useful, although it is not necessarily the center, nor a boundary, of the adventitia. Nevertheless, a polynomial, exponential, or any other suitable mathematical function may be used to fit the lateral locations of pixels. The curve fit may also be accomplished by a piecewise fitting of the brightest pixel positions distributed along the longitudinal direction. Other curve fits may be made over the same domain using some other criterion for selecting the pixels in the range of the curve. In some embodiments, a first, second, or third order polynomial may be selected to piecewise curve fit the adventitia datum along segments of the longitudinal extent of the image. Other functions may be used for piecewise or other curve fits of pixels meeting selected criteria over the domain of interest.
In certain embodiments, a lumen datum may be located by one of several methods. In one embodiment, the lumen datum is found by translating the adventitia datum to a location in the lumen at which substantially every pixel along the curve shape has an intensity less than some threshold value. The threshold value may be a lowest intensity of the image. Alternatively, a threshold value may be something above the lowest intensity of pixels in the image, but nevertheless corresponding to the general regional intensity or a bounding limit thereof found within or near the lumen. The lowest intensity of the image may be extracted from a histogram of pixel intensities within a measurement region. In some embodiments, the threshold is set as the intensity of the lowest intensity pixel in the measurement region plus 10 percent of the difference in intensities between the highest and lowest intensities found in the measurement region. In still other embodiments, an operator may simply specify a threshold.
In another embodiment, the lumen datum may be identified by locating the pixel having a lowest intensity proximate some threshold value or below some threshold value. This may be further limited to a circumstance where the next several pixels transversely are likewise of such low intensity in a lateral (vertical, transverse) direction away from the adventitia. By whichever means it is found, a lumen datum comprises a curve fit of pixels representing a set of pixels corresponding to some substantially minimal intensity according to a bounding condition.
In certain embodiments, a media datum may be defined or located by fitting yet another curve to the lateral position of media dark pixels distributed in a longitudinal direction, substantially between the lumen datum and the adventitia datum. Media dark pixels have been found to evidence a local minimal intensity in a sequential search of pixels in a lateral direction, between the lumen datum and the adventitia datum. That is, image intensity tends to increase initially with distance from the lumen, then it tends to decrease to a local minimum within the media, then it tends to increase again as one moves from the media toward the adventitia.
As a practical matter, threshold values of intensity or distance may be provided to limit ranges of interest for any search or other operation using image data. For example, it has been found that a threshold value of ten percent of the difference, between the maximum intensity in a measurement region and the minimum intensity, added to the minimum intensity is a good minimum threshold value for assuring that media dark pixels found are not actually located too close to the lumen. Similarly, a threshold may be set below the maximum intensity within the measurement region, in order to assure that minima are ignored that may still be within the region of non-interest near the adventitia when searching for the location of media dark pixels. In some instances, 25 percent of the difference between maximum and minimum intensities added to the minimum intensity is a good increment for creating a threshold value.
In some circumstances, a pixel located within or at half the distance between (from) the adventitia datum and (to) the lumen datum may be used as the location of a media dark pixel, such as a circumstance where no adequate local minimum is found. That is, if the actual intensities are monotonically decreasing from the adventitia toward the lumen, then no local minimum may exist short of the lumen. In such a circumstance, limiting the media datum points considered to those closer to the adventitia than to the halfway point between the lumen datum and the adventitia datum has been shown to be an effective filter.
In general, the media datum is curve fit to the line of media dark pixels. However, it has also been found effective to establish a temporary curve fit of media dark pixels and move all media dark pixels lying between the temporary curve fit and the adventitia datum directly over (laterally) to the temporary curve fit. By contrast, those media dark pixels that may lie toward the lumen from the temporary curve fit are allowed to maintain their actual values. One physical justification for this filtering concept is the fact that the boundary of the adventitia is not nearly so subject to variation as the noise of data appears to show. Accordingly, and particularly since the actual media/adventitia boundary is of great importance, weighting the media datum to be fit to no points between the temporary curve fit and the adventitia datum, has been shown to be an effective filter.
In certain embodiments, the lumen/intima boundary may be determined by locating the largest local intensity gradient, that is, locating the maximum rate of change in intensity with respect to movement or position in the lateral direction in a traverse from the lumen datum toward the media datum. This point of local steepest ascent in such a lateral traverse has been found to accurately represent the lumen/intima boundary. A spike removing operation may be applied to a lumen/intima boundary to remove aberrant spikes in the boundary. The resulting boundary may also be curve fit to reduce error. In some embodiments the a spike removing operation is performed before any curve fit to improve the accuracy of the resulting curve.
Similarly, the media/adventitia boundary has been found to be accurately represented by those points or pixels representing the point of steepest ascent in intensity or most rapid change in intensity with respect to a lateral position, in a traverse from the media datum toward the adventitia datum. Clearly, the distance between the lumen/intima boundary and the media/adventitia boundary represents the intima-media thickness. A spike removing operation may be applied to a media/adventitia boundary to remove aberrant spikes in the boundary. The resulting boundary may also be curve fit to reduce error. In some embodiments the a spike removing operation is performed before any curve fit to improve the accuracy of the resulting curve.